Fabrication of a circuit module with a coaxial transmission line

ABSTRACT

Interconnections are made through a planar circuit by a monolithic short-circuited transmission path which extends from a circuit portion of the planar circuit to the opposite side. The opposite side is ground sufficiently to remove the short-circuiting plate, thereby separating the previously monolithic conductors, and exposing ends of the separated conductors of the transmission path. Connection is made between the exposed conductors of the transmission path and the registered contacts of a second planar circuit by means of electrically conductive, compliant fuzz buttons. The transmission path may be a coaxial path useful for RF.

This application is a division of application Ser. No. 09/070,033, filedApr. 30, 1998.

FIELD OF THE INVENTION

This invention relates to RF (including microwave) interconnectionsamong layers of assemblies of multiple integrated circuits, and moreparticularly to interconnection arrangements which may be sandwichedbetween adjacent circuits.

BACKGROUND OF THE INVENTION

Active antenna arrays are expected to provide performance improvementsand reduce operating costs of communications systems. An active antennaarray includes an array of antenna elements. In this context, theantenna element may be viewed as being a transducer which convertsbetween free-space electromagnetic radiation and guided waves. In anactive antenna array, each antenna element, or a subgroup of antennaelements, is associated with an active module. The active module may bea low-noise receiver for low-noise amplification of the signal receivedby its associated antenna element(s), or it may be a power amplifier foramplifying the signal to be transmitted by the associated antennaelement(s). Many active antenna arrays use transmit-receive (T/R)modules which perform both functions in relation to their associatedantenna elements. The active modules, in addition to providingamplification, ordinarily also provide amplitude and phase control ofthe signals traversing the module, in order to point the beam(s) of theantenna in the desired direction. In some arrangements, the activemodule also includes filters, circulators, andor other functions.

A major cost driver in active antenna arrays is the active transmit orreceive, or T/R module. It is desirable to use monolithic microwaveintegrated circuits (MMIC) to reduce cost and to enhance repeatabilityfrom element to element of the array. Some prior-art arrangements useceramic-substrate high-density-interconnect (HDI) substrate for theMMICs, with the substrate mounted to a ceramic, metal, or metal-matrixcomposite base for carrying away heat. These technologies are effective,but the substrates may be too expensive for some applications.

FIG. 1 illustrates a cross-section of an epoxy-encapsulated HDI module10 in which a monolithic microwave integrated circuit (MMIC) 14 ismounted by way of a eutectic solder junction 16 onto the top of aheat-transferring metal deep-reach shim 18. The illustrated MMIC 14,solder 16, and shim 18 are encapsulated, together with other like MMIC,solder and shim assemblies (not illustrated) within a plasticencapsulant or body 12, the material of which may be, for example, epoxyresin. The resulting encapsulated part, which may be termed“HDI-connected chips” inherently has, or the lower surfaces are groundand polished to generate, a flat lower surface 12 _(ls). The flat lowersurface 12 _(ls), and the exposed lower surface 18 _(ls) of the shim,are coated with a layer 20 of electrically and thermally conductivematerial, such as copper or gold. As so far described, the module 10 ofFIG. 1 has a plurality of individual MMIC mounted or encapsulated withinthe plastic body 12, but no connections are provided between theindividual MMICs or between any one MMIC and the outside world. Heatwhich might be generated by the MMIC, were it operational, would flowpreferentially through the solder junction 16 and the shim 18 to theconductive layer 20.

In FIG. 1, the upper surface of MMIC 14 has two representativeelectrically conductive connections or electrodes 14 ₁ and 14 ₂.Connections are made between electrodes 14 ₁ and 14 ₂ and thecorresponding electrodes (not illustrated) of others of the MMICs (notillustrated) encapsulated within body 12 by means of HDI technology,including flexible layers of KAPTON on which traces or patterns ofconductive paths, some of which are illustrated as 32 ₁ and 32 ₂, havebeen placed, and in which the various layers are interconnected by meansof conductive vias. In FIG. 1, KAPTON layers 24, 26, and 30 are providedwith paths defined by traces or patterns of conductors. The layersillustrated as 24 and 26 are bonded together to form a multilayer,double-sided structure, with conductive paths on its upper and lowersurfaces, and additional conductive paths lying between layers 24 and26. Double-sided layer 24/26 is mounted on upper surface 12 _(us) ofbody 12 by a layer 22 of adhesive. A further layer 30 of KAPTON, withits own pattern of electrically conductive traces 32 ₂, is held to theupper surface of double-sided layer 24/26 by means of an adhesive layer28. The uppermost layer of electrically conductive traces may includeprinted antenna elements in one embodiment of the invention. Asmentioned above, electrical connections are made between the conductivetraces of the various layers, and between the traces and appropriateones of the MMIC contacts 14 ₁ and 14 ₂, by through vias, some of whichare illustrated as 36. The items designated MT0, MT1, MT2, and MT3 atthe left of FIG. 1 are designations of various ones of the flexiblesheets carrying the various conductive traces. Those skilled in the artwill recognize this structure as being an HDI interconnectionarrangement, which is described in U.S. Pat. No. 5,552,633, issued Sep.3, 1996 in the name of Sharma.

As illustrated in FIG. 1, at least one radio-frequency (RF) groundconductor layer or “plane” 34 is associated with lower layer 24 of thedouble-sided layer 24/26. Those skilled in the art will realize that thepresence of ground plane 34 allows ordinary “microstrip”transmission-line techniques to carry RF signals in lateral directions,parallel with upper surface 12 _(us) of plastic body 12, so that RFsignals can also be transmitted from one MMIC to another in the assembly10 of FIG. 1.

Allowed U.S. patent application Ser. No. 08/815,349, in the name ofMcNulty et al., describes an arrangement by which signals can be coupledto and from an HDI circuit such as that of FIG. 1. As described in theMcNulty et al. application, the HDI KAPTON layers with their patterns ofconductive traces are lapped over an internal terminal portion of ahermetically sealed housing. Connections are made within the body of thehousing between the internal terminal portion and an externallyaccessible terminal portion.

One of the advantages of an antenna array is that it is a relativelyflat structure, by comparison with the three-dimensional curvature ofreflector-type antennas. When assemblies such as that of FIG. 1 are tobe used for the transmit-receive modules of an active array antenna, itis often desirable to keep the structure as flat as possible, so as, forexample, to make it relatively easy to conform the antenna array to theouter surface of a vehicle. FIG. 2a illustrates an HDI module such asthat described in the abovementioned McNulty patent application. In FIG.2a, representative module 210 includes a mounting base 210, to whichheat is transferred from internal chips. A plurality of mounting holesare provided, some of which are designated 298. A contoured lid 213 ishermetically sealed to a peripheral portion of base 212, to protect thechips within. A first set of electrical connection terminals, some ofwhich are designated as 222 a, 224 a, and 226 a are illustrated as beinglocated on the near side of the base, and a similar set of connectionterminals, including terminals designated as 222 b, 224 b, and 226 b arelocated on the remote side of the base. FIG. 2b is a perspective orisometric view, partially exploded, of an active array antenna 200. InFIG. 2b, the rear or reverse side (the non-radiating or connection side)of a flat antenna element structure 202 is shown, divided into rowsdesignated a, b, c, and d and columns 1, 2, 3, 4, and 5. Each locationof array structure 202 is identified by its row and column number, andeach such location is associated with a set of terminals, three innumber for each location. Each array location of antenna element array202 is associated with an antenna element, which is on the obverse orfront side of structure 202. Each antenna element on the obverse side ofthe antenna element structure 202 is connected to the associated set ofthree terminals on the corresponding row and column of the reverse sideof the antenna element array 202. Each antenna element of active antennaarray 200 of FIG. 2b is associated with a corresponding active antennamodule 210, only one of which is illustrated. In FIG. 2b, active antennamodule 210 b 3 is associated with antenna element or array element 202 b3. Active module 210 b 3 is identical to module 210 of FIG. 2a and toall of the other modules (not illustrated) of FIG. 2b. Representativemodule 210 b 3 has its terminals 222 a, 224 a, and 226 a connected bymeans of electrical conductors to the set of three terminals associatedwith array element 202 b 3 of antenna structure 202. The other set ofterminals of module 210 b 3, namely the set including terminals 222 b,224 b, and 226 b, is available to connect to a source or sink of signalswhich are to be transmitted or received, respectively. It will be clearthat the orientation of module 210 b 3, and of the other modules whichit represents, will, when all present, will extend for a significantdistance behind or to the rear of the antenna element support structure202, thereby tending to make the active antenna array 200 fairly thick.Also, the presence of the many modules will make it difficult to supportthe individual modules in a manner such that heat can readily beextracted from the mounting plates (212 of FIG. 2a). Also, the presenceof many such active modules 210 will make it difficult to make theconnections between the terminal sets of the active modules and theterminal sets of the antenna elements. The problem of thickness of thestructure of FIG. 2b is exacerbated by the need for a signaldistribution arrangement, partially illustrated as 290. Distributionarrangement 290 receives signal from a source 292, and distributes someof the signal to the near connections of each of the modules, such asconnections 222 b. 224 b, and 226 b of module 210 b 3.

A further problem with the structure of FIG. 2b is that the connectionsbetween the active module 210 b 3 and the set of terminals for arrayelement 202 b 3 is by way of an open transmission-line. Those skilled inthe art of RF and microwave communications know that such opentransmission-lines tend to be lossy, and in a structure such as thatillustrated in FIG. 2b, the losses will tend to result in cross-couplingof signal between the terminals of the various array elements.

A further problem with interconnecting the structure of FIG. 2b is thatof tolerance build-up between the antenna terminal sets on the reverseside of the antenna element structure 202, the terminals of the modules210, and the terminals of beamformer 290.

Improved arrangements are desired for producing flat HDI-connectedstructures which can be arrayed with other flat structures.

SUMMARY OF THE INVENTION

A short-circuited transmission line according to an aspect of theinvention includes a monolithic, electrically conductive structureincluding (a) a solid center conductor having a circular cross-sectionabout a central axis. The center conductor terminates at a first planeand has a first diameter at the first plane in a direction transverse tothe central axis, and a second diameter, greater than the firstdiameter, at a second plane parallel to the first plane. The diameter ofthe center conductor tapers monotonically between the first and seconddiameters. The length of the center conductor is defined by theseparation of the first and second planes. The monolithic structurefurther includes (b) a plurality of mutually identical solid outerconductors. Each one of the outer conductors has a circularcross-section about a longitudinal axis. The longitudinal axes of theouter conductors are parallel with the central axis of the centerconductor. Each of the outer conductors terminates at the first plane,and has a third diameter at the first plane, and a fourth diameter,greater than the third diameter, at the second plane. The diameter ofthe outer conductors tapers monotonically between the first and seconddiameters. The outer conductors have their longitudinal axes equallyspaced from each other at radii which make equal angles with adjacentradii. The monolithic structure also includes (c) a solidshort-circuiting plate interconnecting the center conductor and theouter conductors at the second plane.

In a particular embodiment of the invention, the third diameter equalsthe first diameter, and the fourth diameter equals the second diameter,and the taper of the diameters of the center and outer conductors islinear. In another embodiment of the invention, the short-circuitingplate has a thickness no greater than the length of the centerconductor. The periphery of the short-circuiting plate may be defined bya radius measured from the central axis of the center conductor, whichradius is equal to the sum of (a) one of the radii plus (b) half of thegreater of (i) the second diameter and (ii) the fourth diameter. In yetanother embodiment, the length of the center conductor is no greaterthan the diameter of the dielectric insulator.

In one embodiment, a disk-like dielectric insulator encapsulates themonolithic structure. The insulator defines a central axis coincidentwith the central axis of the center conductor, a thickness sufficient toenclose that portion of the center and outer conductors lying betweenthe first and second planes, and a periphery defined, at least in part,by a radius from the central axis sufficient to encapsulate the sides atthe greatest taper, which is a radius which is greater than the sum of(a) one of the radii plus (b) half of the greater of (i) the seconddiameter and (ii) the fourth diameter. In one embodiment of theinvention, the second and fourth diameters are equal, so the radius ofthe encapsulating insulator is equal to the radius of the circles onwhich the outer conductor axes lie, plus half the diameter of aconductor at the second plane. The insulator surrounds at least portionsof the center and outer conductors, for insulating the center conductorfrom the outer conductors and the outer conductors from each other,except at the short-circuiting plate. The dielectric insulator may beeither rigid or deformable, as an elastomer.

A method for producing a flat antenna array according to another aspectof the invention includes the step of affixing a plurality of microwaveintegrated-circuit chips to a planar support, with connections of thechips adjacent to the support. A short electrical transmission-line isprocured. The electrical transmission-line includes

(i) a monolithic, electrically conductive structure which includes

(a) a solid center conductor having a circular cross-section about acentral axis, and terminating in a first end at a first plane. Thecenter conductor has a first diameter at a first plane transverse to thecentral axis, and a second diameter, greater than the first diameter, ata second plane parallel to the first plane. The diameter of the centerconductor tapers monotonically between the first and second diameters.The length of the center conductor is defined by the separation of thefirst and second planes,

(b) a plurality of mutually identical solid outer conductors. Each oneof the outer conductors has a circular cross-section about alongitudinal axis, and the longitudinal axes of the outer conductors lieparallel with the central axis of the center conductor. Each of theouter conductors terminates at a first end at the first plane, and thefirst ends of the outer conductors have a third diameter at the firstplane. The outer conductors have a fourth diameter, greater than thethird diameter, at the second plane. The diameter of the outerconductors tapers monotonically between the third and fourth diameters.The outer conductors have their longitudinal axes equally spaced fromeach other at radii which make equal angles with adjacent radii.

(c) a solid short-circuiting plate interconnecting the center conductorand the outer conductors at the second plane.

In a particular embodiment of the invention, the electrical transmissionline also includes

(ii) a disk-like dielectric insulator defining a central axis coincidentwith the central axis of the center conductor, and a thicknesssufficient to enclose that portion of the center and outer conductorslying between the first and second planes. The periphery of thedisk-like dielectric insulator is defined, at least in part, by a radiusfrom the central axis which is sufficient to enclose the all the outerconductors at their greatest diameter. This radius is no less than thesum of (a) one of the radii plus (b) half of the greater of (i) thesecond diameter and (ii) the fourth diameter. The insulator, when used,surrounds at least portions of the center and outer conductors in theaxial direction, for insulating the center conductor from the outerconductors and the outer conductors from each other, but no electricalinsulation between the conductors exists at the short-circuiting plate.

The method includes the step of applying the short transmission-line tothe support with the first ends adjacent the support, and encapsulatingthe chips and the short transmission-line in rigid dielectric material,to thereby produce a structure including an encapsulated chip andtransmission-line. At least portions of the support are removed from theencapsulated chip and transmission-line, to thereby expose at leastportions of a first side of the encapsulated chip and transmission-line,including at least the connections of the chips and the first ends ofthe center and outer conductors of the short transmission-line. If thesupport lacks conductive traces, a layer of flexible dielectric sheetcarrying a plurality of electrically conductive traces is applied to thefirst side of the encapsulated chip and transmission-line. At least oneof the connections of at least one of the chips is interconnected withthe first end of the center conductor of the transmission-line, and atleast one other of the connections of the one of the chips isinterconnected to the first ends of all of the outer conductors of thetransmission-line, by way of some of the traces and through vias, tothereby produce a first-side-connected encapsulated arrangement. Atleast so much material is removed from that side of thefirst-side-connected encapsulated arrangement which is remote from thefirst side as will expose separated second ends, remote from the firstends, of the center and outer conductors of the transmission-line, tothereby produce a first planar arrangement having exposed second ends ofthe center and outer conductors of the transmission-line. A planarconductor arrangement including a plurality of individual electricalconnections is applied over the first planar arrangement, adjacent theside of the first planar arrangement with exposed second ends of thecenter and outer conductors. The electrical connections of the planarconductor are selected so that, when the planar conductor arrangement isregistered with the first planar arrangement, the electrical connectionsare registered with the center and outer conductors of thetransmission-line. The planar conductor arrangement is registered withthe first planar arrangement, and electrical connections are madebetween the second ends of the center and outer conductors of thetransmission line of the first planar arrangement and the connections ofthe planar conductor arrangement.

In a particular method according to an aspect of the invention, the stepof making electrical connections includes the steps of placing acompressible floccule of electrically conductive material between thesecond ends of each of the center and outer conductors of thetransmission line of the first planar arrangement and the registeredones of the electrical connections of the planar conductor arrangement,and compressing the compressible floccule of electrically conductivematerial between the second ends of the center and outer conductors ofthe transmission lines of the first planar arrangement and theregistered ones of the electrical connections of the planar conductorarrangement, to thereby establish the electrical connections and to aidin holding the compressible floccules in place. In a preferredembodiment of the invention, the method encapsulates the chips and theshort transmission-line in the same dielectric material used in thedielectric disk.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a simplified cross-sectional view of a portion of a prior-arthigh-density interconnect arrangement by which connections are madebetween multiple integrated-circuit chips mounted on a single supportingsubstrate;

FIG. 2a is a simplified perspective or isometric view of a prior-artmodule which contains HDI-connected integrated-circuit chips, and FIG.2b illustrates how a flat or planar antenna array might use a pluralityof the modules of FIG. 2a to form an active antenna array;

FIGS. 3a and 3 b are simplified plan and elevation views, respectively,of a short transmission-line, and FIG. 3c is a cross-section of thestructure of FIG. 3a taken along section lines 3 c- 3 c;

FIGS. 4a, 4 b, 4 c, 4 d, 4 e, 4 f, 4 g, and 4 h illustrate steps, insimplified form, in the fabrication of an RF HDI structures using ashort transmission-line as in FIGS. 3a, 3 b, and 3 c to interface toanother planar circuit, illustrated as a beamformer or manifold;

FIG. 5 illustrates an arrangement similar to that of FIG. 4h with a coldplate interposed between the HDI-connected chips and the beamformer, andusing a rigid fuzz button holder;

FIG. 6a is a simplified plan view of a compressible or conformable shorttransmission line, FIG. 6b is a simplified cross-section of thearrangement of FIG. 6a taken along section lines 6 a- 6 a, FIG. 6c is asimplified perspective or isometric view of the short transmission lineof FIGS. 6a and 6 b, with the fuzz button conductors illustrated inphantom, and FIG. 6d is a simplified perspective or isometric view of arepresentative fuzz button;

FIG. 7 is a simplified cross-sectional representation of an assemblageincluding a cold plate, in which a compressible fuzz button holder isused;

FIG. 8 is a simplified perspective or isometric view, exploded to revealcertain details, of the assemblage of FIG. 7;

FIG. 9a is a simplified perspective or isometric view of ashort-circuited transmission line according to an aspect of theinvention, FIG. 9b is a side or elevation view of the transmission lineof FIG. 9a, FIG. 9c illustrates the arrangement of FIG. 9a inencapsulated form, and FIG. 9d is a side elevation of the encapsulatedstructure of FIG. 9c;

FIG. 10a illustrates the result of certain fabrication stepscorresponding to the steps of FIGS. 4a, 4 b, 4 c, and 4 d applied to theshort-circuited transmission line of FIGS. 9c and 9 d, and FIG. 10billustrates the result of further fabrication steps applied to thestructure of FIG. 10a;

FIG. 11 illustrates a short-circuited multiple transmission line whichmay be encapsulated as described in conjunction with FIGS. 9c or 9 d,and used for interconnecting planar circuit arrangements at frequenciessomewhat lower than the higher RF frequencies, such as the clockfrequencies of logic circuits;

FIG. 12 is a perspective or isometric view of a structure according toan aspect of the invention, including a planar plastic HDI circuit, abipartite separator plate, and a second planar circuit, some of whichare cut away to reveal interior details;

FIG. 13 is an exploded view of the structure of FIG. 12, showing theplanar plastic HDI circuit associated with one portion of the separatorplate as one part, the second portion of the separator plate, and thesecond planar circuit as other parts of the exploded structure;

FIG. 14 is an exploded view of a portion of the second part of theseparator plate, showing rigid and compliant transmission lines, andother structure; and

FIG. 15 is a more detailed cross-sectional view of the structure of FIG.12.

DESCRIPTION OF THE INVENTION

In FIGS. 3a 3 b, and 3 c, a short transmission line or “molded coaxialinterconnect” 310 is in the form of a flat disk or right circularcylinder 311 having a thickness 312 and an outer diameter 314 centeredabout an axis 308. Thickness 312 should not exceed diameter 314. Anelectrically conductive center conductor 316 is in the form of a rightcircular cylinder defining a central axis which is concentric with axis308. A set 318 of a plurality, in this case eight, of further electricalconductors 318 a, 318 b, 318 c, 318 d, 318 e, 318 f, 318 g, and 318 h,are also in the form of right circular cylinders, with axes which lieparallel with the axis 308 of the flat disk. The further electricalconductors have their axes equally spaced by an incremental angle of 45°on a circle of diameter 320, also centered on axis 308. The main body ofshort transmission line 310 is made from a dielectric material, whichencapsulates the sides, but not the ends, of center conductor 316 andouter conductors 318 a, 318 b, 318 c, 318 d, 318 e, 318 f, 318 g, and318 h. The diameter of circle 320 on which the axes of the outerconductors lie is selected so that the outer conductors lie completelywithin the outer periphery of the dielectric disk. A first end of thecenter conductor and the outer conductors lies adjacent a plane 301, anda second end of each lies adjacent to a second plane 302. In aparticular embodiment of the short transmission line, the thickness 312is 0.055 in., and the diameter is 0.304 in. In another embodiment, thediameter is the same, but the thickness is 0.115 in. In bothembodiments, the axes of the outer conductors of set 308 are centered ona circle of diameter 0.192 in., and the conductors have diameters of0.032 in. The material of the dielectric disk is Plaskon SMT-B-1 moldingcompound, and the conductors are copper. As described below, these shorttransmission lines are used for interconnecting RF circuits. Thecharacteristic impedance of the short transmission line of FIGS. 3a, 3b, and 3 c is selected to substantially match the impedances of thesignal source and sink, or to substantially match the impedances of thestripline or microstrip transmission lines to which the shorttransmission line is connected in an HDI circuit. The impedance Z₀ ofthe short transmission line is determined by $\begin{matrix}{Z_{0} = {\left( \frac{138}{\sqrt{ɛ}} \right){\log_{10}\left( \frac{D_{0}}{D_{i}} \right)}}} & 1\end{matrix}$

where

∈ is the dielectric constant of the dielectric disk;

D_(o) is the diameter of the inside surface of the outer conductor; and

D_(i) is the outer diameter of the center conductor. To produce a 50-ohmcharacteristic impedance, with center conductor wire diameter of 0.032″and epoxy encapsulation material having a dielectric constant of 3.7,the axes of the outer conductors should be on a circle having a diameterof 0.192 inches.

FIGS. 4a, 4 b, 4 c, 4 d, 4 e, 4 f, and 4 h illustrate steps in thefabrication of an RF HDI structure. In a step preceding that illustratedin FIG. 4a, one or more short transmission lines 310 are fabricated, andmonolithic RF circuits 14 are assembled with heat-transferring metaldeep-reach shims 18. In FIG. 4a, the chip/shim assemblages 14/18 and theshort transmission lines 310 are mounted face-down onto an adhesivebacked KAPTON substrate 410. FIG. 4b illustrates the encapsulation ofthe assemblages 14/18 and the short transmission line 310 within anepoxy or other encapsulation to form a structure with encapsulated chipsand transmission-lines. The structure of FIG. 4b with encapsulated chipsand transmission lines then continues through conventional HDIprocessing. As illustrated in FIG. 4c, vias are laser-drilled to diebond pads 14 ₁ and 14 ₂ and to the conductors of the short transmissionline 310 which are against the substrate 410. Conductive traces are thenpatterned on the exposed substrate 410, making the necessary electricalconnections. FIG. 4d illustrates the result of applying a plurality(illustrated as three) of layers of conductive-trace bearing flexibleHDI connection material designated together as 424, with the tracesappropriately registered with the connections 14 ₁ and 14 ₂ of the chips14, and with the center conductor 316 and the set 318 of outerconductors of the short transmission line 310.

Following the step illustrated in FIG. 4c, plated through-vias 36 areformed in the conductive-trace bearing flexible HDI connection material424, with the result that the chip connections are made, and theconnections to the short transmission line 18 are made as illustrated inFIG. 4e. The metallization layers 32 connect the short transmission lineto at least one of the chips 14, so that one connection of a chipconnects to center conductor 316 of short transmission line 310 of FIG.4e, and so that a ground conductor associated with the chip connects tothe set 318 of outer conductors of the short transmission line. FIG. 4frepresents the cutting off of that portion of the encapsulated structure(the structure of FIG. 4e) which lies, in FIG. 4f, above a dash line426. This produces a planar structure 401, illustrated in FIG. 4g, inwhich the connections among the chips 14, and between the chips and oneend of the short transmission lines, lie within the conductive-tracelayers 424 on the “bottom” of the encapsulated structure, and in which aheat interface end 18 _(hi) of each of the heat-conducting shims 18, andthe ends of the center conductor 316 and of the set 318 of outerconductors of a coaxial connection structure 490 at the end of the shorttransmission line, are exposed on the “upper” side of the structure ascontacts. The center conductor contact is illustrated as 316 _(c), andsome of the outer conductor contacts are designated as 318 a _(c) and318 f _(c).

FIG. 4h illustrates a cross-section of a structure resulting from afurther step following the step illustrated in conjunction with FIGS. 4fand 4 g. More particularly, the structure of FIG. 4g is attached to anRF manifold or beamformer 430, which distributes the signals which areto be radiated by the active array antenna. The surface 430 s ofmanifold 430 which is adjacent to the encapsulated structure bearsconductive traces, some of which are designated 432. In order to makecontact between the conductive traces 432 on the RF distributionmanifold and the exposed ends of the center conductor 316 and the set318 of outer conductors of the short transmission line, compressibleelectrical conductors 450, termed “fuzz buttons,” are placed between theconductive traces 432 on the distribution manifold 430 and the exposedends of the center conductor 316 and set 318 of outer conductors of eachof the short transmission lines 310. The manifold 430 is then pressedagainst the remainder of the structure, with the fuzz buttons between,which compresses the fuzz buttons to make good electrical connection tothe adjacent surfaces, and which also tends to hold the fuzz buttons inplace due to compression. Appropriate thermal connection must also bemade between the manifold and the shims 18 to aid in carrying away heat.Thus, in the arrangement of FIGS. 4a- 4 h, electrical RF signals aredistributed to the ports (only one illustrated) of the distributionmanifold 430 to a plurality of the ports (only one of which isillustrated) represented by short transmission lines 310 of planarcircuit 401 of FIG. 4g, and the signals are coupled through the shorttransmission lines to appropriate ones of the metallization layers 32 ₀,32 ₁, and 32 ₂, as may be required to carry the signals to the MMIC foramplification or other processing, and the signals processed by the MMICare then passed through the signal paths defined by the paths defined byconductive traces 32 ₀, 32 ₁, and 32 ₂ to that layer of conductivetraces which is most remote from the distribution manifold 430. Moreparticularly, when the distribution manifold 430 is in the illustratedposition relative to the encapsulated pieces, the uppermost layer 32 ₂of conductive traces may itself define the antenna elements. Thus, thestructure 400 defined in FIG. 4h, together with other portions whichappear in other ones of FIGS. 4a- 4 g, comprises the distribution,signal processing, and radiating portions of a planar or flat activearray antenna.

The fuzz buttons 450 of FIG. 4h may be part no. 3300050, manufactured byTECKNIT, whose address is 129 Dermodry Street, Cranford, N.J. 07016,phone (908) 272-5500.

If the conductors 32 ₂ of metallization layer MT2 of FIG. 4h areelemental antenna elements, the RF manifold 430 can be a feeddistribution arrangement which establishes some measure of control overthe distribution of signals to the active MMICs of the various antennaelements. On the other hand, the structure of FIG. 4h denominated as RFmanifold 430 could instead be an antenna array, with the elementalantennas on side 430 p, while the metallization layers 32 ₁ and 32 ₂would in that case distribute the signals to be radiated, or collect thereceived signals. Thus, the described structure is simply a connectionarrangement between two separated planar distribution sets.

It will be noted that in FIG. 4h, the region 460 about the fuzz buttons450 is surrounded by air dielectric, which has a dielectric constant ofapproximately 1. Since the fuzz buttons 450 have roughly the samediameter as the center conductor 316 and the outer conductors 318, thecharacteristic impedance of the section 460 of transmission lineextending from exposed traces 432 to short transmission line 310 islarger than that of the short transmission line. If the shorttransmission line has a characteristic impedance of about 50 ohms, thecharacteristic impedance of the region 460 will be greater than 50 ohms.Those skilled in the art know that such a change of impedance has theeffect of interposing an effective inductance into the transmissionpath, and may be undesirable.

FIG. 5 represents a structure such as that of FIG. 4h with a cold plate510 interposed between the HDI-connected chips 10 on structure 12 andthe beamformer 430. The cold plate 510 has an interface surface 510 iswhich makes contact with the adjacent surface of the plastic body 12 ofthe HDI circuit 10. The cold plate may be, as known in the art, a metalplate with fluid coolant channels or tubes located within, for carryingheat from heat interface surfaces 18 _(hi) to a heat rejection location(not illustrated). Those skilled in the art know that a heat conductivegrease or other material may be required at the interface. Asillustrated in FIG. 5, a fuzz button housing 512 has a thickness aboutequal to that of the cold plate, for holding fuzz buttons 450 in acoaxial pattern similar to that of center conductor 316 and outerconductors 318, for making connections between the center conductor316/outer conductors 318 and the corresponding metallizations 432 of thebeamformer 430. More particularly, the outer conductors 318 and theouter conductor fuzz buttons 450 lie on a circle with diameter d192. Thedielectric constant of the material of fuzz button housing 512 isselected to provide the selected characteristic impedance. As alsoillustrated in FIG. 5, fuzz button housing 512 is not quite as large indiameter as the cut-out or aperture in cold plate 510, in order to taketolerance build-up. Consequently, an air-dielectric gap 512 _(g1) existsaround fuzz button housing 512. The axial length of fuzz button housing512 is similarly not quite as great as the thickness of the cold plate510, resulting in a gap 512 _(g2). Gaps 512 _(g1) and 512 _(g2) have aneffect on the characteristic impedance of the transmission path providedby the fuzz buttons 450 which is similar to the effect of the air gap460 of FIG. 4h. In an analysis of an arrangement similar to that of FIG.5, the calculated through loss was 0.8 dB, and the return loss was only10.5 dB.

The fuzz button housing or holder 512 is made from an elastomericmaterial, which compresses when compressed between the HDI-connectedchips 10 and the underlying beamformer 430, so as to eliminate air gapswhich might adversely affect the transmission path. FIGS. 6a, 6 b, and 6c are views of a compressible or compliant RF interconnect with fuzzbutton conductors. In FIGS. 6a, 6 b, and 6 c, elements corresponding tothose of FIGS. 3a, 3 b, and 3 c are designated by like referencenumerals, but in the 600 series rather than in the 300 series. Asillustrated in FIGS. 6a, 6 b, and 6 c, compliant RF interconnect 610includes a fuzz button center conductor 616 defining an axis 608, and aset 618 including a plurality, illustrated as eight, of fuzz buttonouter conductors 618 a, 618 b, 618 c, 618 d, 618 e, 618 f, 618 g, and618 h, spaced at equal angular increments, which in the case of eightouter conductor elements corresponds to 45°, about center axis 608, on aradius 620 having a diameter of 0.200″. Dielectric body 611 has an outerperiphery 611 p, and is made from a silicone elastomer having adielectric constant within the range of 2.7 to 2.9, and has an overalldiameter 614 of about 0.36″, and a thickness 612 of 0.10″. As can bebest seen in FIGS. 6a and 6 c, the dielectric body 611 has two keyingnotches 650 a and 650 b. Dielectric body 611 also has a flanged innerportion 648 with a diameter of 0.30″, and the maximum-diameter portion652 has a thickness 654 of about 0.44″. The fuzz buttons 616, 618 have alength 613 in the axial direction which is slightly greater (0.115″ inthe embodiment) than the axial dimension 612 of body 611 (0.10″). FIG.6d illustrates a representative one of the outer conductor fuzz buttons,which is selected to be fuzz button 618 f for definiteness. In FIG. 6d,outer conductor fuzz button 618 f is in the form of a right circularcylinder centered on an axis 617, and defines first and second ends 618f ₁ and 618 f ₂ which are coincident with planes 601 and 602,respectively, of FIG. 6b. The cylindrical form of fuzz button 618 f ofFIG. 6d defines an outer surface 618 _(fs) lying between the first andsecond ends 618 f ₁ and 618 f ₂.

FIG. 7 is similar to FIG. 5, and corresponding elements are designatedby the same reference numerals. In FIG. 7, the compliant RF interconnect610 is compressed between the broad surface 430 _(fs), of beamformermanifold 430 and the broad surface 712 _(ls), of HDI-connected chiparrangement 10, and is somewhat compressed axially, to thereby eliminatethe gap 512 _(g2) which appears in FIG. 5. This, in turn, eliminates theprincipal portion of the impedance discontinuity at the interface whichis filled by the compliant RF interconnect 610. The axial compression ofthe dielectric body 611 of the compliant RF interconnect 610, in turn,tends to cause the compliant body 611 to expand radially, to therebysomewhat fill the circumferential or annular gap 512 _(g1), whichfurther tends to reduce impedance discontinuities at the interface. Afurther advantage of the axial compression of body 611 is that thecompression tends to compress the body 611 around the fuzz buttonconductors 616, 618, to help in holding them in place. Analysis of thearrangement of FIG. 7 indicated that the through loss would be 0.3 dBand the return loss 28 dB, which is much better than the values of 0.8dB and 10.5 dB calculated for the arrangement of FIG. 5.

As illustrated in FIG. 7, a heat-transfer interface surface 18 _(hi) onthe broad surface 712 _(ls), of HDI-connected chip structure 10 ispressed against cold plate 510.

In the view of FIG. 7, the fuzz button conductors 616 and 618 of thecompliant coaxial interconnect 610 are illustrated as being of adifferent diameter than the conductors 316, 318 of the molded coaxialinterconnect 310, and the outer conductors 618 are centered on a circleof somewhat different diameter than the outer conductors 318. Thedifference in diameter of the wires and the spacing of the outerconductor from the axis of the center conductor is attributable todifferences in the dielectric constant of the epoxy which is used as thedielectric material in the molded coaxial interconnect 310 and thesilicone material which is the dielectric material of compliantinterconnect 610. In order to minimize reflection losses, bothinterconnects are maintained near 50 ohms, which requires slightlydifferent dimensioning. This should not be a problem, so long as thediameters of the circles on which the outer conductors of the molded andcompliant interconnects are centered allow an overlap of the conductivematerial, so that contact is made at the interface.

A method for making electrical connections as described in conjunctionwith FIGS. 6a, 6 b, 6 c, 7, and 8 includes the step of providing orprocuring a first planar circuit 10 including at least a first broadsurface 712 _(ls). The first broad surface 712 _(ls) of the first planarcircuit 10 includes at least one region 490 defining a first coaxialconnection. It may also include at least a first thermally conductiveregion 18 _(hi) to which heat flows from an active device within thefirst planar circuit. The first coaxial connection 490 of the firstplanar circuit 10 defines a center conductor contact 616 _(c) centeredon a first axis 608 orthogonal to the first broad surface of the firstplanar circuit 10, and also defines a first plurality of outer conductorcontacts, such as 618 a _(c) and 618 f _(c). Each of the outer conductorcontacts such as 618 a _(c), 618 f _(c) of the first coaxial connection490 of the first planar circuit 10 is centered and equally spaced on acircle spaced by a first particular radius, equal to half of diameterd192, from the first axis 608 of the center conductor contact 616 of thefirst coaxial connection 490. The first broad surface 712 _(ls) of thefirst planar circuit 10 further includes dielectric materialelectrically isolating the center conductor contact 616 _(c) of thefirst planar circuit 10 from the outer conductor contacts, such as 618 a_(c), 618 f _(c), and the outer conductor contacts, such as 618 a _(c),618 f _(c), from each other. The method also includes the step ofproviding a second planar circuit 430, which includes at least a firstbroad surface 430 _(fs). The first broad surface 430 _(fs) of the secondplanar circuit 430 includes at least one region 431 defining a coaxialconnection. The coaxial connection 431 of the second planar circuit 430includes a center conductor contact ⁴³² _(c) centered on a second axis808 orthogonal to the first broad surface 430 _(fs) of the second planarcircuit 430, and also includes the first plurality (eight) of outerconductor contacts 432 _(o). Each of the outer conductor contacts, suchas 432 _(co), 432 _(o), of the coaxial connection 431 of the secondplanar circuit 430 is centered and equally spaced on a circle spaced bya second particular radius, close in value to the first particularradius, from second axis 808 of the center conductor contact 432 _(c) ofthe coaxial connector 431 of the second planar circuit 430. The firstbroad surface 430 _(fs) of the second planar circuit 430 furtherincludes dielectric material electrically isolating the center conductorcontact 432 _(c) of the second planar circuit 430 from the outerconductor contacts, such as 432 _(co), 432 _(o) of the second planarcircuit 430, and the outer conductor contacts, such as 432 _(co), 432_(o) of the second planar circuit 430, from each other. A compliantcoaxial connector 610 is provided, which includes (a) a center conductor616 which is electrically conductive and physically compliant, at leastin the axial direction. The compliant center conductor 616 has the formof a circular cylinder centered about a third axis 608, and defines anaxial length 613 between first 617 _(f1), and second 617 _(f2) ends. Thecompliant coaxial connector 610 also includes (b) an outer electricalconductor arrangement 618 including a set 618 including the firstplurality (eight) of mutually identical, electrically conductive,physically compliant outer conductors 618 a, 618 b, 618 c, 618 d, 618 e,618 f, 618 g, and 618 h. Each of the compliant outer conductors 618 a,618 b, 618 c, 618 d, 618 e, 618 f, 618 g, and 618 h is in the form of acircular cylinder centered about an axis 617, and each has an axiallength 613 between first 617 _(f1) and second 617 _(f2) ends which isequal to the axial length 613 of the compliant center conductor 616. Theaxes 617 of the compliant outer conductors 618 a, 618 b, 618 c, 618 d,618 e, 618 f, 618 g, and 618 h are oriented parallel with each other,and with the third axis 608 of the compliant center conductor 616. Thefirst ends 617 _(f1) of the compliant center conductor 616 and thecompliant outer conductors 618 a, 618 b, 618 c, 618 d, 618 e, 618 f, 618g, and 618 h coincide with a first plane 601 which is orthogonal to theaxes 608, 617 of the compliant center conductor 616 and the compliantouter conductors 618 a, 618 b, 618 c, 618 d, 618 e, 618 f, 618 g, and618 h, and the second ends 617 _(f2) of the compliant center conductor616 and the compliant outer conductors 618 a, 618 b, 618 c, 618 d, 618e, 618 f, 618 g, and 618 h coincide with a second plane 602 parallelwith the first plane 601. The compliant outer conductors 618 a, 618 b,618 c, 618 d, 618 e, 618 f, 618 g, and 618 h have their axes 617 equallyspaced from each other at the particular radius from the axis 608 of thecompliant center conductor 616. The compliant coaxial connector 610further includes (c) a compliant dielectric disk-like structure 611defining a fourth center axis 608 coincident with the third axis 608 ofthe compliant center conductor 616 and also defining an uncompressedaxial length no more than about 10% greater than the uncompressed axiallength of the compliant center conductor 616. The compliant disk-likestructure 611 also defines a periphery 611 p spaced from the center axis608 by a second radius which is greater than both (a) the first radius(half of diameter 620) and (b) the axial length 613 of the compliantcenter conductor 616. The compliant dielectric disk 611 surrounds andsupports the compliant center conductor 616 and the compliant outerconductors 618 a, 618 b, 618 c, 618 d, 618 e, 618 f, 618 g, and 618 h atleast on side regions 618 _(fs) thereof lying between the first 618_(f1) and second 618 _(f2) ends of the compliant center conductor 616and the compliant outer conductors 618 a, 618 b, 618 c, 618 d, 618 e,618 f, 618 g, and 618 h. The compliant dielectric disk-like structure611 does not overlie the first 618 _(f2) ends of the compliant centerconductor 616 and the compliant outer conductors 618 a, 618 b, 618 c,618 d, 618 e, 618 f, 618 g, and 618 h, so that electrical connectionthereto can be easily established.

The method described in conjunction with FIGS. 6a, 6 b, 6 c, 7, and 8also includes the further step of placing the first broad surfaces 712_(ls), 430 _(fs) of the first and second planar circuits 10, 430mutually parallel, with the first axis 8 passing through the center ofthe center conductor contact 316 c of the first planar circuit 10 andorthogonal to the first broad surface 712 _(ls) of the first planarcircuit 10, and coaxial with the second axis 808 passing through thecenter of the center conductor contact 432 _(c) of the second planarcircuit 430 orthogonal to the first broad surface 430 _(ls) of thesecond planar circuit 430, with the first and second planar circuits 10,430 rotationally oriented around the coaxial first and second axes 8,808 so that a fourth axis 880 orthogonal to the first broad side 712_(ls) of the first planar circuit 10 and passing through the center ofone of the outer conductor contacts 318 _(cc) of the first coaxialconnector 431 of the first planar circuit 10 is coaxial with a fifthaxis 882 orthogonal to the first broad side 430 _(fs) of the secondplanar circuit 430 and passing through the center of one of the outerconductor contacts 432 _(cc) of the first coaxial connector 431 of thesecond planar circuit 430. The compliant coaxial connector 310 is placedbetween the first and second planar circuits 10, 430, with the thirdaxis 608 of the compliant center conductor 616 substantially coaxialwith the mutually coaxial first and second axes 8, 808. The compliantcoaxial connector 610 is oriented so that a sixth axis 884 of one of thecompliant outer conductors 618 a, 618 b, 618 c, 618 d, 618 e, 618 f, 618g, and 618 h is coaxial with the mutually coaxial fourth and fifth axes880, 882. Force is applied to translate the first and second planarcircuits 10, 430 toward each other until the compliant coaxial connector610 is compressed between the first broad surface 712 _(ls) of the firstplanar circuit 10 and the first broad surface 430 _(fs) of the secondplanar circuit 430 sufficiently to make contact between the centerconductor contacts 316 _(c), 432 _(c) of the first and second planarcircuits 10, 430 through the compliant center conductor 616, and to makecontact between outer conductor contacts 318 a _(c), 318 f _(c) of thefirst planar circuit and corresponding outer conductor contacts 432 a_(c), 432 f _(c) of the second planar circuit 430 through some of thecompliant outer conductors 618.

In a particular version of the method described in conjunction withFIGS. 6a, 6 b, 6 c, 7, and 8 also includes the further step of procuringa first planar circuit 10 in which the first broad surface 712 _(ls)includes a first thermally conductive region 18 _(hi) to which heatflows from an active device within the first planar circuit. In thisversion of the method, before the step of applying force to translatethe first and second planar circuits 10, 430 toward each other, a planarspacer or cold plate 510 is interposed between the first broad surface712 _(ls) of the first planar circuit 10 and the first broad surface 430_(fs) of the second planar circuit 430. In this method, the step ofinterposing a planar cold plate 510 between the first broad surfaces 712_(ls), 430 _(fs) comprises the step of interposing a planar cold plate510 having an aperture 810 with internal dimensions no smaller thantwice the second radius of the compliant dielectric disk-like structure610, with the outer periphery of the aperture 810 surrounding thecompliant coaxial connector 610.

FIG. 9a is a simplified perspective or isometric view of a shortmonolithic (one-piece without joints) conductive short-circuitedtransmission line or RF interconnect 900 according to an aspect of theinvention, FIG. 9b is a side or elevation view of the transmission lineof FIG. 9a, and FIGS. 9c and 9 d illustrate the arrangement of FIG. 9ain encapsulated form. In FIGS. 9a and 9 b, the short-circuitedtransmission line or RF interconnect 900 has an air dielectric, and ismade by machining from a block, or preferably by casting. Transmissionline 900 includes a center conductor 916 centered on an axis 908, andhaving a circular cross-section. Center conductor 916 ends at a plane903 in a flat circular end 916 e, and each of the outer conductors 918a, 918 b, 918 c, 918 d, 918 e, 918 f, and 918 h also has a correspondingflat circular end 918 ae, 918 be, 918 ce, 918 de, 918 ee, 918 fe, and918 he. The cross-sectional diameters of the center conductor 916 andthe outer conductors 918 a, 918 b, 918 c, 918 d, 918 e, 918 f, and 918 htaper from a relatively small diameter d_(i) of the circular ends atplane 903 to a larger diameter d₂ at a second plane 902. At (orimmediately adjacent to) plane 902, a short-circuiting plate 907interconnects the ends of the center conductor 916 and the outerconductors 918 a, 918 b, 918 c, 918 d, 918 e, 918 f, and 918 h which areremote from plane 903. In FIGS. 9a and 9 b, the axes of outer conductors918 a, 918 b, 918 c, 918 d, 918 e, 918 f, and 918 h, only one of whichis illustrated and designated 918 aa, lie on a circle illustrated as adash line 921, which lies at a radius 920 from axis 908 of centerconductor 916. The periphery lip of short-circuiting plate 907 isillustrated as being circular, with a diameter or radius measured fromaxis 908 which is just large enough so that the outer edges of thevarious outer conductors of set 918 are coincident or tangent withperiphery lip at plane 902.

While not the best mode of using the short-circuited transmission lineof FIGS. 9a and 9 b, FIGS. 9c and 9 d illustrate the short-circuitedtransmission line 900 of FIGS. 9a and 9 b encapsulated in a cylindricalbody 911 of dielectric material corresponding to the dielectric body 311of FIG. 3, to form an encapsulated short-circuited transmission line901. As illustrated in FIG. 9c, the encapsulating body 911 does notcover the ends 916 e and 918 ae, 918 be, 918 ce, 918 de, 918 ee, 918 fe,and 918 he of the center and outer conductors, thereby making themavailable for connections. As also illustrated in FIG. 9c, the diameterof dielectric body 911 of encapsulated short-circuited transmission line901 is the same as the diameter 914 of the short-circuiting plate 907,so the side of the short-circuiting plate 907 is exposed. The diameterof the dielectric encapsulating body could be greater than diameter 914of the short-circuiting plate 907, in which case the plate 907 would notbe visible in FIG. 9c.

With the unencapsulated short-circuited transmission-line 900 made asdescribed in conjunction with FIGS. 9a, 9 b, or with the encapsulatedshort-circuited transmission line 901 made as described in conjunctionwith FIGS. 9a, 9 b, 9 c, and 9 d, the unencapsulated (900) orencapsulated transmission line (901) can then be made a part of a planarcircuit. The unencapsulated short-circuited transmission line 900 ofFIGS. 9a and 9 b, or the encapsulated transmission line 901, is placedon a substrate 410 as illustrated for circuit 310 in FIG. 4a, with itsexposed conductor ends 916 e, 918 ae, 918 be, 918 ce, 918 de, 918 ee,918 fe, and 918 he adjacent substrate 410. The steps of FIGS. 4b, 4 c,and 4 d are followed.

FIG. 10a is a simplified representation of the result of applying thesteps of FIGS. 4a, 4 b, 4 c, and 4 d to the encapsulated transmissionline 901 of FIGS. 9a, 9 b, and 9 c. In FIG. 10a, elements correspondingto those of FIG. 4e are designated by like reference numerals, andelements corresponding to those of FIGS. 9a, 9 b, 9 c, and 9 d aredesignated by like reference numerals. As illustrated in FIG. 10a, theplanar circuit structure 1000, which may be an antenna array, has thelocation of the short-circuiting plate 907 below the parting plane 426at which a cut is made to expose a newly formed end 1016 e of thetapered center conductor and to also expose newly formed ends of the setof outer conductors 918, respectively. As illustrated in FIG. 10a, theparting plane lies between planes 903 and 902 associated with the RFinterconnect 900. FIG. 10b is a simplified cross-section of a structuregenerally similar to that of FIG. 4h, in which the structure of FIG. 10ais the starting point; elements of FIG. 10b corresponding to those ofFIG. 10a are designated by like reference numerals, and elementscorresponding to those of FIG. 4h are designated by like referencenumerals. It will be apparent to those skilled in the art that thestructure of FIG. 10B is equivalent to that of FIG. 4h, with the soledifference lying in the tapered diameter of the center conductor 916 andof the outer conductors represented by 918 b and 918 f between the smallends 916 e and newly formed large ends 1018 be and 1018 fe,respectively. This taper may change the characteristic impedancesomewhat between the ends of the RF interconnect, but this effect ismitigated by the relatively small taper, and because the axial length ofthe RF interconnect is selected to be relatively short in terms ofwavelength at the highest frequency of operation. Naturally, if one ormore unencapsulated short-circuited transmission lines 900 are used tomake the planar circuit according to the method described in conjunctionwith FIGS. 4a, 4 b, 4 c, 4 d, 10 a, and 10 b, the dielectric constant ofthe encapsulant material of the transmission line is the same as that ofthe planar circuit itself. If an encapsulated transmission line such as901 is used to make the planar circuit of FIG. 10b, it is desirable thatthe encapsulating materials be identical.

FIG. 11 illustrates a monolithic electrically conductive structure whichforms multiple short-circuited transmission paths, each consisting of atleast one conductor paired with another; as known to those skilled inthe art, one of the pair may be common with other circuit paths, and maybe used at somewhat lower frequencies than the coaxial structures, downto zero frequency. In FIG. 11, the multiple short-circuited transmissionpaths take the form of a monolithic electrically conductive structure1110, including a baseplate 1112 and a plurality, eleven in number, oftapered pins or posts 1114 a, 1114 b, 1114 c, 1114 d, 1114 e, 1114 f,1114 g, 1114 h, 1114 i, 1114 j, and 1114 k. The short-circuited multipletransmission-line structure is used instead of the coaxial arrangement900 in the method described in conjunction with FIGS. 4a, 4 b, 4 c, 4 d,10 a, and 10 b, to make a planar structure. Those skilled in the artknow that antenna array/beamformer combinations require not onlyconnection of RF signals, but also require transmission between elementsof power and control signals, which can be handled by the structure madewith the multiple transmission paths of FIG. 11.

FIGS. 12, 13, 14, and 15 illustrate a planar plastic HDI circuit 10similar to those described in conjunction with FIGS. 3a, 3 b, 3 c, 4 a,4 b, 4 c, 4 d, 4 e, 4 f, and 4 g. More particularly, planar plastic HDIcircuit 10 includes a molded interconnect 310 such as that described inconjunction with FIGS. 3a, 3 b, and 3 c, assembled to the substrate 12as described in conjunction with FIGS. 4a, 4 b, 4 c, 4 d, 4 e, 4 f, and4 g. The planar plastic HDI circuit 10 is mounted on a stiffening plate510 a, which is part of a bipartite separation plate 510. First portion510 a of the bipartite separation plate 510 has an aperture 810 formedtherein to accommodate the flanged disk-like body of compliantinterconnect 610, with the fuzz-button conductors 616, 618 of thecompliant interconnect registered with the conductors of moldedinterconnect 310 so as to be in contact therewith.

Second portion 510 b of separation plate 510 of FIGS. 12, 13, 14, and 15has a through aperture 1312 including a cylindrical portion, and alsoincluding a recess 1214 ₂ adjacent side 1310 b of second portion 510 bof separation plate 510, which recess accommodates a hold-down flange1214. Through aperture 1312 also includes a lip or flange 1314 adjacentside 1310 c, which aids in holding the body of a rigid coaxialtransmission line 1210 in place. Rigid coaxial transmission line 1210 issimilar to molded interconnect 310, but may be longer, so as to be ableto carry signals through the first and second portions of the separationplate 510. Aperture 1312 also defines a key receptacle 1316 whichaccepts a key 1212 protruding from the body of rigid transmission line1210. The number of conductors of rigid transmission line 1210 isselected, and the conductors are oriented about the longitudinal axis ofthe rigid transmission line, in such a manner as, when keyed into theaperture 1312 in separation plate 510, the conductors each match andmake contact with corresponding conductors of compliant interconnects610 a and 610 b. Compliant interconnect 610 a is compressed betweenmolded interconnect 310 and rigid coaxial transmission line 1210, and isoriented to make the appropriate connections between the center fuzzbutton 616 of molded interconnect 610 a and the center conductor 1210 c,and between the outer fuzz buttons 618 of molded interconnect 610 a andthe outer conductors, one of which is designated 1210 o, of the rigidtransmission line 1210.

Molded interconnect 610 b of FIGS. 12, 13, 14, and 15 is compressedbetween a surface 1210 s of rigid transmission line 1210 and face 430 sof second circuit 430, and, when the second circuit 430 is registeredwith separation plate 510, the center and outer metallizations 1332 and1334, respectively, of its coaxial port 1331 are registered with thecorresponding center fuzz button 616 and outer fuzz buttons 618 ofcompliant interconnect 610 b. The second compliant interconnect 610 b isheld in place by flange 1214, which in turn is held down by screws 1216a and 1216 b in threaded apertures 1218 a and 1218 b, respectively.

It will be clear from FIGS. 12, 13, 14, and 15 that when the center axis308 of the center-conductor connection 316 c of port 490 of the HDIcircuit 10 are coaxial with the axis 1308 of the center-conductorconnection 1332 of the port 1331 of the beamformer or second circuit430, and with the axes 1408, 1210 cca, and 1432 ca of the centerconductors of the first compliant interconnect 610 a, the rigidtransmission line 1210, and the second compliant interconnect 610 b, andthe compliant interconnects are of sufficient length, an electricallycontinuous path will be made between the two center conductor contacts.Similarly, with the center conductors and center conductor contactscoaxial, all that is required to guarantee that the outer conductorsmake corresponding contact is that they have the same number and beequally spaced about the center conductors, and that one of the outerconductors or outer conductor contacts in each piece lie in a commonplane with the common axes of the center conductors. When any one of theeight outer conductors or contacts of any one of the interconnectionelements is aligned with the corresponding others, all of the outerconductors or outer conductor contacts is also aligned with itscorresponding elements.

In the particular embodiment of the invention illustrated in FIGS. 12,13, 14, and 15, the separation plate 510 consists of a stiffener plate510 a which is adhesively or otherwise held to the otherwise flexibleplastic HDI circuit 12, and the second portion 510 b of separator plate510 is a cold plate, which includes interior chambers (not illustrated)into which chilled water or other coolant may be introduced by pipesillustrated as 1230 a and 1230 b. In a particular embodiment of theinvention, the planar plastic HDI circuit (only a portion illustrated)defines an antenna array, and the MMIC (not illustrated in FIGS. 12, 13,14, and 15) associated with the planar plastic HDI circuit include chipsoperated as active amplifiers for the antenna elements. The secondcircuit 430 is part of a beamformer which supplies signals to, andreceives signals from, the MMIC associated with the planar plastic HDIcircuit 12.

Other embodiments of the invention will be apparent to those skilled inthe art. For example, while the described flat antenna structure lies ina plane, it may be curved to conform to the outer contour of a vehiclesuch as an airplane, so that the flat antenna structure takes on athree-dimensional curvature. It should be understood that an activeantenna array may, for cost or other reasons, define element locationswhich are not filled by actual antenna elements, such an array is termed“thinned.” The term “RF” has been used to indicate frequencies which maymake use of the desirable characteristics of coaxial transmission lines;this term is meant to include all frequencies, ranging from a fewhundred kHz to at least the lower infrared frequencies, about 10¹³ Hz.,or even higher if the physical structures can be made sufficientlyexactly. While the short transmission line illustrated in FIGS. 3a, 3 b,and 3 c has eight outer conductors, the number may greater or lesser.The dielectric constant of the dielectric conductor holder of the shorttransmission lines is selected to provide the proper impedance, whereasthe specified ranges are suitable for 50 ohms. While the cold plate hasbeen described as being for carrying away heat generated by chips in thefirst planar circuit 10, it will also carry away heat from thedistribution beamformer. While the diameters of the center and outerconductors have been illustrated as being equal, the center conductormay have a different diameter or taper than the outer conductors, andthe outer conductors may even have different diameters among themselves.

Thus, a short-circuited transmission line (900) according to an aspectof the invention includes a monolithic, electrically conductivestructure including (a) a solid center conductor (916) having a circularcross-section about a central axis (908). The center conductor (916)terminates at a first plane (901) and has a first diameter (d1) at thefirst plane (901) in a direction transverse to the central axis (908),and a second diameter (d2), greater than the first diameter (d1), at asecond plane (902) parallel to the first plane (901). The diameter ofthe center conductor (916) tapers monotonically between the first (d1)and second (d2) diameters. The length of the center conductor (916) isdefined by the separation of the first (901) and second (902) planes.The monolithic structure further includes (b) a plurality (eight, in theillustrated embodiment) of mutually identical solid outer conductors(918 a, 918 b, 918 c, 918 d, 918 e, 918 f, 918 g, and 918 h). Each oneof the outer conductors (918 a, 918 b, 918 c, 918 d, 918 e, 918 f, 918g, and 918 h) has a circular cross-section about a longitudinal axis(such as axis 910 aa). The longitudinal axes (such as 918 aa) of theouter conductors (918 a, 918 b, 918 c, 918 d, 918 e, 918 f, 918 g, and918 h) are parallel with the central axis (908) of the center conductor(916). Each of the outer conductors (918 a, 918 b, 918 c, 918 d, 918 e,918 f, 918 g, and 918 h) terminates at the first plane (901), and has athird diameter (d3) at the first plane (901), and a fourth diameter(d4), greater than the third diameter (d3), at the second plane (902).The diameter of each of the outer conductors (918 a, 918 b, 918 c, 918d, 918 e, 918 f, 918 g, and 918 h) tapers monotonically between thefirst (d1) and second (d2) diameters. The outer conductors (918 a, 918b, 918 c, 918 d, 918 e, 918 f, 918 g, and 918 h) have their longitudinalaxes (such as 918 aa) equally spaced from each other at radii (920) fromthe center axis (908) which make equal angles (45° in the case of eightouter conductors) with adjacent radii (920). The monolithic structurealso includes (c) a solid short-circuiting plate (907) interconnectingthe center conductor (916) and the outer conductors (918 a, 918 b, 918c, 918 d, 918 e, 918 f, 918 g, and 918 h) at the second plane (902).

In one embodiment of the short-circuited transmission line, a disk-likedielectric insulator (911) encapsulates the monolithic structure. Theinsulator (911) defines a central axis (908) coincident with the centralaxis (908) of the center conductor (916), and also defines a thickness(t2) sufficient to enclose that portion of the center and outerconductors (918 a, 918 b, 918 c, 918 d, 918 e, 918 f, 918 g, and 918 h)lying between the first (901) and second (902) planes, and furtherdefines a periphery (911 p), at least in part, by a radius (998) fromthe central axis (908) sufficient to encapsulate the sides of the outerconductors at their largest diameter, which is a radius (998) which isgreater than the sum of (a) one of the radii (920) plus (b) half of thegreater of (i) the second diameter (d2) and (ii) the fourth diameter(d4).

In one embodiment of the invention, the second (d2) and fourth diameters(d4) are equal, so the radius (998) of the encapsulating insulator (911)is equal to the radius of the circle (921) on which the axes (such as918 aa) of the outer conductors (918 a, 918 b, 918 c, 918 d, 918 e, 918f, 918 g, and 918 h) lie, plus half the diameter (d2) of a conductor atthe second plane (902). The insulator (911) surrounds at least portionsof the center (916) and outer conductors (918 a, 918 b, 918 c, 918 d,918 e, 918 f, 918 g, and 918 h), for insulating the center conductor(916) from the outer conductors (918 a, 918 b, 918 c, 918 d, 918 e, 918f, 918 g, and 918 h) and the outer conductors (918 a, 918 b, 918 c, 918d, 918 e, 918 f, 918 g, and 918 h) from each other, except at theshort-circuiting plate (907). In a particular embodiment of theinvention, the third diameter (d3) equals the first diameter (d), andthe fourth diameter (d4) equals the second diameter (d2), and the taperof the diameters of the center and outer conductors (918 a, 918 b, 918c, 918 d, 918 e, 918 f, 918 g, and 918 h) is linear. In anotherembodiment of the invention, the short-circuiting plate (907) has athickness (t) no greater than the length of the center conductor (916).The periphery of the short-circuiting plate (907) may be the same asthat of the insulator (911), which is to say that it is defined by aradius measured from the central axis (908) of the center conductor(916), which radius is equal to the sum of (a) one of the radii (920)plus (b) half of the greater of (i) the second diameter (d2) and (ii)the fourth diameter (d4). In yet another embodiment, the length of thecenter conductor (916) is no greater than the diameter of the dielectricinsulator (911). The dielectric insulator (911), if used, may be eitherrigid or deformable, as for example an elastomer.

A method for producing a flat circuit structure, which may be an antennaarray, according to another aspect of the invention, includes the stepof affixing a plurality of microwave integrated-circuit chips (14) to aplanar support (410), with connections (14 ₁, 14 ₂) of the chips (14)adjacent to the support (410). A short-circuited electricaltransmission-line is procured. The short-circuited electricaltransmission-line includes

(i) a monolithic, electrically conductive structure (900) which includes

(a) a solid center conductor (916) having a circular cross-section abouta central axis (908), and terminating in a first end (916 e) at a firstplane (901). The center conductor (916) has a first diameter (d1) at afirst plane (901) transverse to the central axis (908), and a seconddiameter (d2), greater than the first diameter (d1), at a second plane(902) parallel to the first plane (901). The diameter of the centerconductor (916) tapers monotonically between the first (d1) and second(d2) diameters. The length of the center conductor (916) is defined bythe separation of the first (901) and second (902) planes,

(b) a plurality (eight) of mutually identical solid outer conductors(918 a, 918 b, 918 c, 918 d, 918 e, 918 f, 918 g, and 918 h). Each oneof the outer conductors (918 a, 918 b, 918 c, 918 d, 918 e, 918 f, 918g, and 918 h) has a circular cross-section about a longitudinal axis(such as axis 918 aa), and the longitudinal axes (such as axis 918 aa)of the outer conductors (918 a, 918 b, 918 c, 918 d, 918 e, 918 f, 918g, and 918 h) lie parallel with the central axis (908) of the centerconductor (916). Each of the outer conductors (918 a, 918 b, 918 c, 918d, 918 e, 918 f, 918 g, and 918 h) terminates at a first end at thefirst plane (901), and the first ends of each of the outer conductors(918 a, 918 b, 918 c, 918 d, 918 e, 918 f, 918 g, and 918 h) have athird diameter (d3) at the first plane (901). The outer conductors (918a, 918 b, 918 c, 918 d, 918 e, 918 f, 918 g, and 918 h) have a fourthdiameter (d4), greater than the third diameter (d3), at the second plane(902). The diameter of the outer conductors (918 a, 918 b, 918 c, 918 d,918 e, 918 f, 918 g, and 918 h) tapers monotonically between the third(d3) and fourth (d4) diameters. The outer conductors (918 a, 918 b, 918c, 918 d, 918 e, 918 f, 918 g, and 918 h) have their longitudinal axes(such as axis 918 aa) equally spaced from each other at radii (920)which make equal angles (45° for the case of eight outer conductors)with adjacent radii (920).

(c) a solid short-circuiting plate (907) interconnecting the centerconductor (916) and the outer conductors (918 a, 918 b, 918 c, 918 d,918 e, 918 f, 918 g, and 918 h) at the second plane (902). Theelectrical transmission line (900) may also include

(ii) a disk-like dielectric insulator (911) defining a central axiscoincident with the central axis (908) of the center conductor (916),and a thickness sufficient to enclose that portion of the center andouter conductors (918 a, 918 b, 918 c, 918 d, 918 e, 918 f, 918 g, and918 h) lying between the first (901) and second (902) planes. Theperiphery (911 p) of the disk-like dielectric insulator (911) isdefined, at least in part, by a radius (998) from the central axis (908)which is sufficient to enclose the all the outer conductors (918 a, 918b, 918 c, 918 d, 918 e, 918 f, 918 g, and 918 h) at their greatestdiameter. This radius (998) is no less than the sum of (a) one of theradii (920) plus (b) half of the greater of (i) the third diameter (d3)and (ii) the fourth diameter (d4). The insulator (911) surrounds atleast portions of the center and outer conductors (918 a, 918 b, 918 c,918 d, 918 e, 918 f, 918 g, and 918 h) in the axial direction, forelectrically insulating the center conductor (916) from the outerconductors (918 a, 918 b, 918 c, 918 d, 918 e, 918 f, 918 g, and 918 h)and the outer conductors (918 a, 918 b, 918 c, 918 d, 918 e, 918 f, 918g, and 918 h) from each other, but no electrical isolation between theconductors exists at the short-circuiting plate (907).

The method includes the step of applying the short-circuitedtransmission line (900) to the support (410) with the first ends (916 e,918 xe, where x ranges from a to h) of the center (916) and outerconductors (918 a, 918 b, 918 c, 918 d, 918 e, 918 f, 918 g, and 918 h)adjacent the support (410), and encapsulating the chips (14) and theshort-circuited transmission line (900) in rigid dielectric material(412), to thereby produce a structure (FIG. 4c) including anencapsulated chip (14) and transmission line (900). At least portions ofthe support (410) are removed from the encapsulated chip (14) andtransmission line (900), to thereby expose at least portions of thefirst side of the encapsulated chip (14) and transmission line (900),including at least the connections (14 ₁, 14 ₂) of the chips (14) andthe first ends (916 e, 918 xe) of the center (916) and outer (918 a, 918b, 918 c, 918 d, 918 e, 918 f, 918 g, and 918 h) conductors of theshort-circuited transmission line (900). At least one of the connections(14 ₁, 14 ₂) of at least one of the chips (14) is interconnected (by wayof paths 32 ₁ and vias 36) with the first end (916 e) of the centerconductor (916) of the short-circuited transmission line (900), and atleast one other of the connections (14 ₁, 14 ₂) of the one of the chips(14) is interconnected to the first ends (918 xe) of all of the outerconductors (918 a, 918 b, 918 c, 918 d, 918 e, 918 f, 918 g, and 918 h)of the transmission-line, by way of conductive traces and through vias,to thereby produce a first-side-connected encapsulated arrangement (FIG.4d, 10 a). This may be accomplished by connecting the traces of a layer(424) of flexible dielectric sheet carrying a plurality of electricallyconductive traces (32 ₁, 322) applied to the first side of theencapsulated chip (14) and short-circuited transmission line (900). Atleast so much material (1010) is removed (FIG. 10a, 10 b) from that sideof the first-side-connected encapsulated arrangement (FIG. 4d) remotefrom the first side as will expose separated second ends (1016 e, 1018xe), remote from the first ends (916 e, 918 xe), of the center (916) andouter conductors (918 a, 918 b, 918 c, 918 d, 918 e, 918 f, 918 g, and918 h) of the transmission line, and to thereby eliminate theshort-circuit, to thereby produce a first planar arrangement (1050 ofFIG. 10b) having exposed second ends (1016 e, 1018 xe) of the center(916) and outer conductors (918 a, 918 b, 918 c, 918 d, 918 e, 918 f,918 g, and 918 h) of the transmission line. A planar conductorarrangement (430) including a plurality of individual electricalconnections (432) is applied over the first planar arrangement (1050),adjacent the side of the first planar arrangement with exposed secondends (1016 e, 1018 xe) of the center (916) and outer conductors (918 a,918 b, 918 c, 918 d, 918 e, 918 f, 918 g, and 918 h). The electricalconnections (432) of the planar conductor arrangement (430) are selectedso that, when the planar conductor arrangement (430) is registered withthe first planar arrangement (1050), the electrical connections (432)are registered with the center (916) and outer conductors (918 a, 918 b,918 c, 918 d, 918 e, 918 f, 918 g, and 918 h) of the transmission line.The planar conductor arrangement (430) is registered with the firstplanar arrangement (1050), and electrical connections (450) are madebetween the second ends (1016 e, 1018 xe) of the center (916) and outerconductors (918 a, 918 b, 918 c, 918 d, 918 e, 918 f, 918 g, and 918 h)of the transmission line (900) of the first planar arrangement (1050)and the connections (432) of the planar conductor arrangement (430).

In a particular method according to an aspect of the invention, the stepof making electrical connections includes the steps of placing acompressible floccule (fuzz buttons) of electrically conductive materialbetween the second ends (1016 e, 1018 xe) of each of the center (916)and outer conductors (918 a, 918 b, 918 c, 918 d, 918 e, 918 f, 918 g,and 918 h) of the transmission line of the first planar arrangement(1050) and the registered ones of the electrical connections (432) ofthe planar conductor arrangement (430), and compressing the compressiblefloccule of electrically conductive material between the second ends(1016 e, 1018 xe) of the center (916) and outer conductors (918 a, 918b, 918 c, 918 d, 918 e, 918 f, 918 g, and 918 h) of the transmissionline of the first planar arrangement (1050) and the registered ones ofthe electrical connections (432) of the planar conductor arrangement(430), to thereby establish the electrical connections and to aid inholding the compressible floccules (450) in place. In a preferredembodiment of the invention, the method encapsulates the chips (14) andthe short-circuited transmission line (900) in the same dielectricmaterial (412) used in the dielectric disk (911).

What is claimed is:
 1. A short-circuited transmission line, comprising:a monolithic, electrically conductive structure including (a) a solidcenter conductor having a circular cross-section about a central axis,said center conductor terminating at a first plane and having a firstdiameter at said first plane transverse to said central axis, and asecond diameter, greater than said first diameter, at a second planeparallel to said first plane, said diameter of said center conductortapering monotonically between said first and second diameters, and thelength of said center conductor being defined by the separation of saidfirst and second planes; (b) a plurality of mutually identical solidouter conductors, each one of said outer conductors having a circularcross-section about a longitudinal axis, said longitudinal axis of eachof said outer conductors being parallel with said central axis of saidcenter conductor, and each of said outer conductors terminating at saidfirst plane and having a third diameter at said first plane, and afourth diameter, greater than said third diameter, at said second plane,said diameter of said outer conductors tapering monotonically betweensaid first and second diameters, and said plurality of outer conductorshaving said longitudinal axes equally spaced from each other at radiiwhich make equal angles with adjacent radii; (c) a solidshort-circuiting plate interconnecting said center conductor and saidouter conductors at said second plane; and a disk-like dielectricinsulator defining a central axis coincident with said central axis ofsaid center conductor, a thickness sufficient to enclose that portion ofsaid center and outer conductors lying between said first and secondplanes, and a periphery defined, at least in part, by a radius from saidcentral axis greater than the sum of (a) one of said radii plus (b) halfof the greater of (i) said second diameter and (ii) said fourthdiameter, said insulator surrounding at least portions of said centerand outer conductors, for insulating said center conductor from saidouter conductors and said outer conductors from each other, except atsaid short-circuiting plate.
 2. A transmission line according to claim1, wherein said third diameter equals said first diameter, and saidfourth diameter equals said second diameter.
 3. A transmission lineaccording to claim 1, wherein said monotonic tapers of said diameter ofsaid center and outer conductors are linear tapers.
 4. A transmissionline according to claim 1, wherein said short-circuiting plate has athickness no greater than said length of said center conductor.
 5. Atransmission line according to claim 1, wherein said short-circuitingplate has a periphery defined by a radius measured from said centralaxis of said center conductor, which radius is equal to the sum of (a)one of said radii plus (b) half of the greater of (i) said seconddiameter and (ii) said fourth diameter.
 6. A transmission line accordingto claim 1, wherein said length of said center conductor is no greaterthan the diameter of said dielectric insulator.
 7. A transmission lineaccording to claim 1, wherein said dielectric insulator is one of arigid and a deformable material.
 8. A transmission line according toclaim 7, wherein said dielectric insulator is made from a deformableelastomer.